The hippocampus is a key cortical structure that plays an important role in a number of normal physiological processes including formation of short- and long-term memory, and is a primary site of pathology in certain neurological disorders including Alzheimer's disease (AD). One of the major neuromodulatory inputs to the hippocampus is a cholinergic projection from the basal forebrain which is critical for memory and attention mechanisms, and degeneration of this projection plays an important role in the pathology of AD. Cholinergic transmission in the hippocampus is mediated primarily by muscarinic acetylcholine receptors (mAChRs), which have been classified into M1 - M5 subtypes. Muscarinic agonists have a number of electrophysiological effects in the hippocampus including potentiation of NMDA receptor currents, reduction of both inhibitory and excitatory synaptic transmission, and direct excitatory effects on pyramidal cells. However, the specific mAChR subtypes involved in each of these actions are unknown. Determination of the specific roles of each of the mAChR subtypes in regulating hippocampal function will be critical for understanding the involvement of mAChR subtypes in both normal and pathological conditions. Until recently, it has not been possible to determine the mAChR subtypes involved in specific responses because of a lack of subtype-selective probes. However, immunocytochemistry studies with antibodies that selectively react with each of the five cloned mAChR subtypes have provided valuable insight that has allowed us to formulate hypotheses regarding the mAChR subtypes that mediate various pre- and postsynaptic effects of mAChR activation. Furthermore, we have now made major advances in discovery of novel highly selective pharmacological reagents that specifically activate either M1 or M4, the major mAChR subtypes in the hippocampus. These reagents, combined with recently developed mAChR knockout mice provide an unprecedented opportunity to rigorously determine the functions of individual mAChR subtypes in the hippocampus and other brain regions. We propose a series of studies in which we will further characterize highly selective activators of M1 and M4 and use these along with mAChR KO mice to determine which mAChR subtypes mediate specific physiological effects of mAChR activation in the hippocampal formation. Agents that selectively enhance cholinergic transmission at specific receptor subtypes could be useful for improving cognitive function in patients with AD and other memory disorders.